1. Field of the Invention
The present invention relates to a method and apparatus for operating a surveillance radar, and in particular to surface detection radar, such as an ASDE (Airport Surface Detection Equipment radar) or a CSR (Coastal Surveillance Radar) which uses a low-cost end-fed, slotted array antenna in combination with frequency agile pulse transmissions for monitoring surface movement of targets. The frequency agile pulse transmissions improve the target detection performance of the radar in the presence of distributed clutter, i.e., rain, hail, etc. Additionally, the invention can reduce the effects of electronic interference at locations having multiple radars, or the intentional electronic jamming of the radar in an electronic warfare environment.
2. Description of the Prior Art
The purpose of surveillance radar, such as an ASDE radar for example, is to provide air traffic controllers of an airport with information on the location of surface traffic. The surface traffic is not confined solely to the moving traffic of aircraft and ground-based vehicles, but also includes parked or stationary aircraft and ground-based vehicles, since information on all surface traffic is essential to an air traffic controller for handling incoming and departing air traffic.
Surface surveillance radars, such as ASDE and CSR radars, are well known to those of ordinary skill in the art, and are described, for example by M. I. Skolnik in the book "Introduction to Radar Systems", 1962. One characteristic of such radars is degradation of target detection performance due to the effects of distributed clutter. In the ASDE case the distributed clutter is usually rain, while in the CSR case the distributed clutter is typically from the surface of the water and is referred to as "sea clutter".
It is known in the design of surveillance radar systems to be advantageous to use radar pulse transmissions of more than one frequency to illuminate a given target detection area. Use of more than one pulse frequency for detecting a target can change the targets' Swerling Case (a classification based on a characteristic fluctuation of the received power in the radar cross-section (RCS) area of the target), and is therefore particularly beneficial for improving target detection performance of the radar system in the presence of distributed clutter. Prior art radars, such as the ASDE-3 radar manufactured and commercially sold by Cardion, Inc of Woodbury, N.Y., typically use several different pulse frequencies for developing return signals representative of a given target detection area. The returns are then combined, resulting in an improvement in the target detection performance of the radar system (i.e., the signal-to clutter S/N ratio of the radar video signal is improved), due to de-correlation of the distributed clutter as compared with correlated returns from a point source reflector (target). However, such prior art radars are much more costly as compare to single frequency radars, since they conventionally use a complex and costly antenna system.
It would be desirable to provide surface detection radar which would allow the use of a lower cost antenna, yet would still allow multiple pulse frequencies to be used for improving the target detection performance of the system in the presence of distributed clutter. One such low-cost antenna is an end-fed slotted array. The design of an end-fed slotted array antenna is a technology that is well understood to those of ordinary skill in the radar technology. The resulting antenna has good gain and beamwidth performance characteristics, and is inherently reasonable in cost to manufacture as compared with large parabolic reflector antennas. Unfortunately, inherent in the design of an end-fed slotted array antenna is the fact that the position of the beam radiated therefrom (in a plane perpendicular to the rotating antenna), varies in angular position ("squints") from a direction which is normal to the radiating surface of the antenna, as a function of the radiated frequency. This phenomena is referred to as frequency squint and the amount of the angular variation is referred to as the squint angle. Due to the frequency dependency of the squint angle, incorporation of such an antenna in a multi-frequency radar in order to reduce the system cost becomes most problematic.
It would also be desirable to provide a surface detection radar which would not only use a lower cost antenna, but would also be frequency agile, that is, easily adjustable to allow multiple and selectable pulse frequencies to be used, for improving the target detection performance of the system in the presence of clutter, and for improving its' target detection performance in the event of either intentional or unintentional electronic interference, i.e., in a multi-radar system or in an electronic warfare environment, respectively.
Additionally, it would be desirable that the cost of making such a frequency agile system not increase the basic manufacturing cost very much as compared to a non-frequency agile system, and in fact it would be desirable for such a system to be of lower manufacturing cost.